Author: Frank Ross
Spotting scopes are, in essence, telescopes with a shorter focal length, designed for terrestrial observations beyond the effective range of typical binoculars. Spotting scopes enable hunters to locate and evaluate game from a great distance, without being detected and give hunters the ability to determine if an animal is worthy of pursuit.
Spotting scopes come in both fixed and variable focal lengths. The variable models are more flexible, enabling wide areas to be scanned with shorter focal lengths and then examined in greater detail at maximum zoom. In order to manage the extreme focal lengths of spotting scopes it is best to use a small, lightweight tripod.
Important issues to consider when selecting a spotting scope are magnification, aperture, field of view, eye relief, and focusing.
Field of View
The widest dimension of circular viewing area observed through a telescope is the field of view. This is normally measured in linear feet at 1000 yards or in angular degrees. Since the field of view normally decreases with increased magnification, this dimension will usually be smaller for a spotting scope than for a binocular, with a typical range of from one degree (52.5 feet) to three degrees (157 feet). At close distances, field of view is much more critical, so this range is quite adequate for the medium to long distance observation. The minimum near focus distance is typically 20 to 30 feet.
Field of view is largely determined by eyepiece design. Some eyepieces are designed to deliver wide fields of view (wide angle) and these are very useful and popular for following a moving object, as in wildlife observation. As a general rule, zoom eyepieces will usually have a more restricted field than an equivalent eyepiece of fixed focal length. Eyepieces designed for long eye relief usually have more narrow fields of view. Field of view can also be related to optical design as in the wide field characteristic of a telescope with a short focal length.
A spotting scope’s aperture is the diameter of the objective lens, usually measured in millimeters. For hunting applications, the aperture size will normally range between 50 to 90mm. While the aperture size will be directly related to the size and weight of the instrument, two other factors, optical design and focal length, have a role as well. The size of the objective lens determines the amount of light that will enter the optical system, with the potential of increased image detail and clarity with increased aperture size. Actual observed image brightness is a function of several factors: the transmittance of the optical system (coatings), the relationship of the aperture to the magnification (exit pupil), eye pupil size, and focal length of the objective lens.
Three different types of focusing mechanisms are used on telescopes. They are helical, rack & pinion, and knob focusers. The characteristics of each type can make a difference in how well it adapts to your specific application.
Helical focusers are knurled or rubberized collars around the scope barrel where the focus is changed fairly rapidly when turning the focusing ring. Helical focusers work well for observing moving objects, near to far, quite rapidly.
Rack & pinion designs are commonly found on astronomical telescopes and usually offer fairly fast, smooth focusing, but most components are external and subject to potential degradation over time from dust and moisture if used in the field. Knob focusers have a slower action but allow for precise, accurate focusing.
The focal length (f.l.) of a scope is simply the distance between the main lens and the point where the light rays from the image come sharply into view. The eyepiece also has a focal length and the combination of these two focal lengths result in the magnification of the image (mag. = f.l. of objective divided by f.l. of eyepiece). Focal length is often expressed in another term, focal ratio, which is the ratio of the aperture diameter to the focal length of the objective lens.
Selecting the right eyepiece or eyepiece configuration is an important consideration when purchasing a spotting scope. The eyepiece magnifies the image of some distant object and delivers this image at a certain distance behind the eyepiece. The diameter of this image is called the exit pupil and eye relief is the distance at which it is located behind the eyepiece. The amount that the image is magnified is determined by the ratio of the focal length of the objective lens (or mirror) to that of the eyepiece (the longer the eyepiece focal length, the lower the magnification for a given aperture).
Eyepieces differ in many ways. Some are fixed in focal length; some are variable (zooms). Others are designed to give either wide fields of view (wide angle) or long eye relief for comfort of those wearing eyeglass. They attach to the scope by different means: direct screw threads, bayonet mounts or by fastening with a setscrew. Some spotting scopes have eyepieces that are not interchangeable. These are usually found on either zooms or waterproof scopes. The eyepiece placement may be configured for straight-through, 45 degree, or 90 degree viewing. Also, some eyepiece designs are available in different diameters, varying from .96" to 1.25" or even as large as 2".
Eyepiece placement is usually a personal preference, and hunters most often prefer the straight-through design. The straight-through design makes it easier to sight an object and follow it as it moves. It is also more convenient to use with a vehicle window mount.
There are basically two types of optical design in spotting scopes: the refractor and the catadioptric. The difference between the two is the material used to focus the image. The refractor uses optical glass lenses to bend light while the catadioptric primarily uses mirrors to reflect the light. The difference in materials is basic, however, each translates into several distinctive optical characteristics.
The refractor design can be as simple as a long tube with an objective lens at one end and an eyepiece at the other. Those designed for hunting use will incorporate some type of image-erecting prism system (either Porro or roof prism) in the optical tube between these two lenses. A prism system provides an image orientation that is correct vertically as well as right to left. For hunting use, the focal length is typically short to provide a compact, lightweight scope with a wide field of view. Eyepieces available for refractor scopes are usually interchangeable, fixed focal lengths and are attached by either the screw or bayonet type mount.
Zoom eyepieces are often available which provide flexibility and convenience in the field at the expense of some loss in brightness, resolution and field of view. These eyepieces normally deliver a magnified image in the range of 15 to 60X, but give the best performance in the 20 to 40X range. Usually the focusing mechanism is of the helical or knob type, which is internal and protected from dust and moisture.
Generally, refractors are made with a simple design and are easy to use, durable, reliable, and require little or no maintenance. Their objective lens is permanently mounted and aligned. Some color aberration may be noticeable with refractors but models using apochromatic (corrected for both chromatic and spherical aberration) lens designs or those with fluorite or extra-low dispersion glass elements, virtually eliminate this problem. Refractors deliver sharp, high contrast images and overall, are the very popular for hunting and general outdoor activities.
The catadioptric design uses a combination of a corrector lens and a pair of mirrors to bring an image into focus. This optical system typically has a long focal length that is folded by mirrors into a compact optical tube. Catadioptics are capable of high magnification and exceptional telephoto lens performance without excessive bulk. Their long focal length, coupled with a large aperture size of typically 90 to 100 mm allows for useful magnifications of up to 200X or more on some models.
Their drawback is inherited in the design and generally limits them to astronomical observations. Long focal length tends to restrict the field of view. Catadioptric scopes are usually supplied with a 45-degree or 90-degree erecting prism that makes them less desirable for hunting applications. The 45-degree prism delivers a correctly oriented image in all dimensions while the image from a 90-degree prism will be erect, but reverted. Both the 45 and 90-degree prisms work well for viewing objects above the horizon, with the 90-degree is more applicable for astronomical observation. Interchangeable fixed-focal-length eyepieces are inserted into the prism to change the magnification.
Since catadioptric scopes operate at higher magnifications with the resulting narrow fields of view, a finder scope is usually supplied for centering the main scope on a distant object.
Author: Frank Ross